HAWTHORNE, California — Prior to revealing it's all-electric big-rig, Tesla showed journalists around the massive vehicle. The company even thoughtfully provided a couple of traditional Class-8 diesel trucks for comparison.

The difference is striking. Because Tesla's designers were able to start from a clean sheet and avoid dealing with the huge amount of space that must be allotted for a diesel engine in a big-rig, the Semi's cabin is more like a space capsule than something B.J. and the Bear would recognize.

"Unlike other trucks, the Semi’s cabin is designed specifically around the driver, featuring unobstructed stairs for easier entry and exit, full standing room inside, and a centered driver position for optimal visibility," Tesla said in a statement.

The cab feels radically different — like the bridge of a starship — but what went into it was familiar.

As Tesla explained, much of the Semi was built from off-the-shelf parts from the Model 3. Development began in 2016, starting from the premise that the company could reinvent the whole idea of a big rig. Four Model 3 drive units would supply the propulsion, with one for each of the rear wheels (two per axle).

The floor of the cab houses the battery pack, while up top, the driver sits in the middle, between a pair of Model 3-sourced touchscreens. The storage areas are vast, and to the right of the driver is a jumpseat for an extra passenger, as well as console containing cupholders and USB ports. Gear can be stowed in pull-down airliner-style overhead bins. There's also a "frunk" — a front trunk — that's big enough to hold a couple of suitcases.

Unlike some long-haul big-rigs, Tesla's Semi doesn't yet have a sleeper cab, but the company said that feature could be added, depending on demand. The battery packs are modular, with the expectation that more or less could be used, based on desired range.

We saw a pair of prototype Semis, one in a shimmery silver, the other in matte black. Each truck has a different aerodynamic element above the cab, for different types of trailers. The rear wheels were also covered on one side, to showcase another aerodynamic feature, and the huge truck wheels had the aerodynamic hubcap-like covers Tesla developed for the Model 3.

In many ways, the Semi is basically a big-ass Model 3 — and while that might sound flippant, using existing Tesla tech was an important aspect of the design process. The goal was to make use of Tesla's future economies of scale with Model 3 components, given that the company could be producing hundreds of thousands of that vehicle in the coming years and could build up a robust supply chain.

That extends to the Autopilot self-driving tech in the Semi, derived from the hardware on the Model 3, but, according to the company, recalibrated for the greater weight and larger size of the truck.

The Apollo-capsule-esque feel of the Semi's cab is also the best evidence yet that what Tesla has learned from creating space capsules at SpaceX, which as it happens is right next door to Tesla's design studio, south of Los Angeles.